1. Field of the Invention
The present invention relates to a coating film forming method and a coating film forming apparatus for forming a coating film by coating a target substrate such as a semiconductor wafer with coating liquid such as a resist solution.
2. Description of the Related Art
In the manufacturing process of, for example, a semiconductor device, a resist pattern is formed as a mask for forming a prescribed pattern by a so-called photolithography technology in which a resist film is formed by supplying a resist solution onto the surface of a semiconductor wafer (hereinafter referred to simply as “wafer”), followed by applying a light exposure treatment to the wafer after the resist coating in conformity with a prescribed pattern and subsequently developing the exposed pattern formed in the resist film on the wafer. In the resist coating process included in the photolithography technology noted above, a spin coating method is employed in many cases as a method for uniformly coating the wafer surface with the resist solution.
In the spin coating method, a wafer fixed on, for example, a spin chuck by the vacuum suction is rotated together with the spin chuck, and a resist solution is allowed to drip from a resist nozzle arranged above the wafer onto substantially the central portion of the wafer. The resist solution dripping onto the wafer surface is centrifugally expanded outward in the radial direction of the wafer, with the result that a resist film is formed on the entire surface of the wafer. Then, the dripping of the resist solution is stopped, and the wafer is kept rotated so as to remove the excess resist solution on the surface of the wafer W so as to control the thickness of the resist film and to dry the resist film.
It should be noted that, in the conventional spin coating method, a resist solution is allowed to drip onto substantially the central portion of the wafer, and the resist solution is expanded by the centrifugal force generated by the rotation of the wafer, as described above. What should be noted is that the peripheral speed in the outer peripheral portion of the wafer is markedly higher than that in the central portion, with the result that a considerably large amount of the resist solution is scattered from the outer peripheral portion of the wafer. It follows that only about 10 to 20% of the supplied resist solution is actually used for forming the resist film, leading to a markedly large consumption of the resist solution required for forming the resist film. Under the circumstances, it is of high importance nowadays to decrease the amount of the resist consumption for the resist coating step, i.e., to decrease the dripping amount of the resist solution onto the wafer, in view of the saving of the manufacturing cost.
As a method for decreasing the resist solution consumption for forming the resist film, proposed in, for example, JP 7-320999 A is a method (pre-wet system) of allowing a solvent such as a thinner to drip onto the substrate prior to the dripping of the resist solution so as to facilitate the diffusion of the resist solution and, thus, to decrease the supply amount of the resist solution.
However, the effect produced by the prior art quoted above differs depending on the kind of the solvent. In other words, a sufficient effect is not necessarily obtained depending on the solvent used. It is certainly possible to overcome this difficulty by selecting an effective solvent. However, the solvent used by the user is limited and, thus, required is a coating film forming method that permits stably decreasing the amount of the coating liquid used such as a resist solution regardless of the kind of the solvent used.